302 Rankvn and M&rwin — 



only incidentally here, 2 but the equilibrium relations of 

 these forms in ternary mixtures of MgO-Al 2 0, r Si0 2 will 

 be considered in detail. On the other hand, cordierite of 

 such composition, and an unstable form having similar 

 composition which appears at lower temperatures, have 

 not been described before," therefore their properties 

 and mode of formation will be taken up. 



General Procedure. 



In this account only such additional apparatus, experi- 

 mental methods and applications of the phase rule will 

 be described as seem to be necessary for understanding 

 this particular problem. 



The first step in this investigation consisted in the 

 preparation from especially pure MgO, A1 2 3 and Si0 2 of 

 a few furnace charges whose composition represented 

 random intervals over the entire range of ternary mix- 

 tures. It may be said concerning these : (1) that compo- 

 sitions high in Si0 2 , being viscous, require repeated 

 fusions with subsequent fine grinding in order to insure 

 chemical homogeneity throughout the charge. This con- 

 dition is established in melts which do not crystallize 

 readily on cooling by the determination of the uniformity 

 in the refractive index of the glass obtained from such 

 melts; (2) that those melts containing so high a percent- 

 age of either MgO, or A1 2 3 that they melt but little if any 

 below the melting point of platinum, 4 about 1750, further 

 heated in the iridium furnace ; this insures the formation 

 of sufficient liquid to permit complete combination to take 

 place ; (3) that in many of the compositions, particularly 

 those high in alumina, the compounds are readily formed 

 from their components at temperatures at which there 

 is but little if any liquid present. 



Having prepared a number of chemically homogenous 

 charges of different compositions, the next step was the 



2 In these melts clinoenstatite is seldom twinned. Needles, prisms and 

 tables have been found in the same quench. 



8 J. Morozewiez, "Experimentelle Untersuehungen iiber die Bildung der 

 Minerale im Magma," p. 68, briefly described crystals obtained from 

 silicate melts, which are similar to this high temperature form, and which 

 he called cordierite. The relation of various anhydrous crystals to cordierite 

 will be discussed under another heading. 



1 All charges are fused in platinum crucibles placed in a Fletcher gas- 

 blast furnace. 



